Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: earl1937 on May 19, 2014, 02:05:53 PM
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:airplane: There are many video's which show flight and or ground crew members, turning props by hand before engine start. There are two reasons for this practice, what are they?
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Making sure the pots aren't hydrolocked and pumping any collected oil out of the bottom pots.
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If the cylinder is hydraulic locked you can not "pump" the oil out by pulling the prop through. More likely you will bend a rod or crack a cylinder. You can turn the prop backwards to suck the oil into the intake but that is frowned on since the oil may just go back into the cylinder on intake.
The correct way to clear the cylinder is too remove the spark plugs and let it drain.
The props are pulled through to detect hydraulic lock. (In theory with modern clutched starters it isn't necessary, the starter clutch should slip if a locked cylinder is encountered)
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Making sure the pots aren't hydrolocked ... ... ... and ... ... ... pumping any collected oil out of the bottom pots.
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It doesn't work that way, you don't pump the oil out if the cylinder is hydraulic locked. You will puke some oil out of the cylinders that have an open valve...but they are not locked so you're not clearing them. Semantics perhaps.
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The props are pulled through to detect hydraulic lock. (In theory with modern clutched starters it isn't necessary, the starter clutch should slip if a locked cylinder is encountered)
Some starters have an input shaft designed to shear under severe load... Don't count on it though, and I'd rather not have to wrestle installing a new starter (and extracting the broken gear shaft). Our 1820s almost never suffered hydraulic lock. Anyway, before starting, it's best to pull 9 blades through. If the prop won't pull, remove the front spark plug and let it drain. When you can pull the prop through one blade, climb in and, mags off, use the starter to pump out residual oil. wipe it down, put the plug back in and crank her up. Still, if the engine has only been sitting for a day or two, you may want to consider what caused so much oil to drain into the lower cylinder.
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It doesn't work that way, you don't pump the oil out if the cylinder is hydraulic locked. You will puke some oil out of the cylinders that have an open valve...but they are not locked so you're not clearing them. Semantics perhaps.
It takes a good deal of oil to hydrolock the cylinder. However there is almost always some oil that needs clearing. By turning the prop this oils is ejected into the exhaust manifold. Which is why there's usually so much smoke on startup if the engine hasn't been run for a while. If the prop comes to a sudden stop and won't move, it is hydrolocked and you have to take out the spark plug in the bottom cylinders to let the collected oil out.
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The oil doesn't need to be "cleared" unless the cylinder is locked. There is no need to pull the prop through to clear oil. The ONLY reason is to ensure a cylinder isn't locked before engaging the starter. Again, unless the cylinder is locked there is NO NEED to clear the oil. But what would I know, I've only started a radial a couple of thousand times, x 4 for pulling the prop through.
FYI, in 2 1/2 years of flying the 17 and 24 we only had a hydraulic lock once, it was on the #2 engine on the B-17 which was a high time engine and puked a lot of oil.
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When you're finished thumping your chest maybe you can get around to answering the second part of Earl's question.
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Oh the second reason....to confuse non pilots. :devil
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Oh the second reason....to confuse non pilots. :devil
:rofl :rofl :rofl
Colmbo bets a gold star..... best answer ever!
:salute
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When you're finished thumping your chest maybe you can get around to answering the second part of Earl's question.
That is not chest thumping. That's a man who has had the privilege of flying the planes in RL that needed the props turned, to check for hydro lock as he stated. Columbo stating "this is how we really did it and why" is the best way to remove any doubt about if he was guessing or actually knew what he is talking about.
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That's a man who has had the privilege of flying the planes in RL that needed the props turned, to check for hydro lock as he stated. Columbo stating "this is how we really did it and why" is the best way to remove any doubt about if he was guessing or actually knew what he is talking about.
...which is way better than reading about it and regurgitating it as expertise.
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Colmbo, your groupies have arrived. :aok
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Since Jumos and DBs were inverted engines, did their props have to be pulled thru. Never seen this mentioned.
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No. The cylinders themselves extended into the crankcase and stuck up above the floor of the case about an inch so that oil didn't run into the cylinders. The only oil that fell into the cylinder was that which dripped off the crankshaft after shutdown, and it wasn't enough to fill the piston to the point that it ran through the oil ring groove drain holes and down past the compression rings into the combustion chamber. Hydraulic lock was never an issue with the DB's or the Jumo's. However there was another issue with the DB 600 series; it was very difficult to obtain consistent oil consumption since the rotation of the crankshaft one cylinder bank got more oil than the other. This increased oil consumption on one cylinder bank and reduced the anti-knock value of the fuel/air charge. For this reason the engine was built with different cylinder compression ratios on the two cylinder banks.
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It's difficult to make out, but on the second cutout from the left you can see the top of the cylinder almost touching a counterweight on the crankshaft.
(http://upload.wikimedia.org/wikipedia/commons/a/a7/Daimler-Benz-DB_601A_Schnitt.jpg)
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It's difficult to make out, but on the second cutout from the left you can see the top of the cylinder almost touching a counterweight on the crankshaft.
(http://upload.wikimedia.org/wikipedia/commons/a/a7/Daimler-Benz-DB_601A_Schnitt.jpg)
:airplane: Good post, but do you know which engine this? The guys might like to know!
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That's a DB 601 (Bf 109E/F, Bf 110C). DB stands for Daimler Benz (same company that make Mercedes cars).
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Well Earl, it doesn't seem like anyone knows both answers to your question. If checking for hydrolocks and pumping out oil is one of the reasons, what is the other? :)
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Well Earl, it doesn't seem like anyone knows both answers to your question. If checking for hydrolocks and pumping out oil is one of the reasons, what is the other? :)
:airplane: Any sign of oil being pumped out thu the exhausts stack system! Most people think that the cloud of smoke, when first cranking an engine, is oil collected in the exhaust "collector" ring! But, not so, smoke comes from oil collecting in bottom cylinders of engine. If you have evidence of oil around the discharge area of the exhaust system, it usually means you have internal problems of some kind in the engine.
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No piston rings make a perfect seal.
This means even a completely filled cylinder can be pulled through............it just takes longer than pulling a spark plug.
Even longer in cold conditions.